1. Field of the Invention
The present invention relates to an improvement of an oxygen sensor, comprising; a sensor element having electrodes on inner and outer surfaces thereof, the electrode on the inner surface communicating to the atmosphere, and the electrode on the outer surface communicating to a gas to be measured; a metallic accommodating member accommodating the sensor element; and at least two airtight sealing portions, arranged in the accommodating member for separating the gas to be measured from the atmosphere.
2. Related Art Statement
Heretofore, the above type of oxygen sensor is structured to separate the atmosphere from a gas to be measured, because the atmosphere is used as a standard oxygen gas. One way of separating the atmosphere from the gas to be measured is shown in FIG. 4, wherein talc powder 8 is filled in spaces defined by a metallic cap 10, a sensor element 12 and ceramic supporters 28a, 28b and 28c, and compressed in axial direction of the oxygen sensor under pressure to form an airtight sealing portion 8-1 at high temperature side of the oxygen sensor and an airtight sealing portion 8-2 at low temperature side of the oxygen sensor.
This type of oxygen sensor has a drawback in that the airtight sealing property of the sealing portions 8-1, 8-2 is gradually deteriorated due to vibrations and/or heat cycles of the oxygen sensor, though the sealing portions 8-1, 8-2 were compressed in axial direction under pressure. As a result, there arises a drawback in that the gas to be measured penetrates or invades in the standard oxygen atmosphere through the airtight sealing portions 8-1, 8-2 finally during a long period of use of the oxygen sensor to decrease the electromotive force of the oxygen sensor.
Regarding the drawback, a counterplan can be considered of increasing the compressing power in the axial direction to the talc powder to enhance the airtight sealing property of the sealing portions. However, in such a case, there arises another drawback in that due to an excessively high compressing power the metallic accommodating member is expanded to render the assembling of the oxygen sensor impossible or the ceramic supporters defining the airtight sealing portions are split.